Svante Pääbo gives a good public lecture on Neanderthals, Denisovans, and other relatives of modern humans

I think most readers know about Svante Pääbo and his work on “paleoanthropology”: the study of the evolution and ancient movements of H. sapiens through analysis of “fossil DNA”. His most famous work is on the genetics of Neanderthals, a subject in which I’ve recently become interested.

Pääbo’s work been extended to Denisovans and other previously unknown human groups, and what we’re learning is that even in H. sapiens the evolutionary tree is convoluted and interconnected. This does not, by the way, vindicate the thesis that evolutionary trees are wrong, or can’t be accurately determined. Despite that, there are some fossil individuals so genetically heterogeneous that they can’t be slotted into one group or another (see below). Our relatives were “mixing” quite promiscuously when they met.

In this remarkably clear lecture (h/t: Matthew Cobb), which proceeds chronologically through the scientific findings, Pääbo lays out the genetic data produced by his lab. (This is the award lecture accompanying Pääbo’s 2018 Nierenberg Award for Science in the Public Interest, given on October 3 of this year.) There’s some freaky stuff in here, including an individual that appears to be an F1 (first-generation) hybrid between a Neanderthal and a Denisovan (about 32 minutes in).

36 minutes into the lecture, Pääbo summaries the contribution of Neanderthal and Denisovan genes to modern humans, including the possibility that a gene we carry from Neanderthals that now gives us a higher propensity for type 2 diabetes could have been an allele that helped Neanderthals deal with starvation. Similarly, Denisovans have bequeathed to modern Tibetan populations a gene that helps deal with high altitude.

In fact, there are at least a dozen “archaic” genes from Neanderthals and Denisovans that remain in our genomes and are associated with disease, perhaps because they don’t function well in the genetic backgrounds of modern humans. (There’s evidence that some of these have been selected against.) At the end, Pääbo discusses the genes in modern humans not present in Denisovans or Neanderthals; the idea here are that these human-specific genes (there are 87) that makes us “important” and “special”. I’ll let you watch those last 12 minutes on your own. There are four minutes of questions at the end.

All in all, this is a superb introduction to the complex and always-changing picture of our relationship to recent hominin relatives. If you watch it, and you should, you’ll be absolutely up to speed on human paleogenetics. But, as Steve Gould used to say, when he lectured on human evolution at Harvard each year, his first act was to throw out all his notes from the previous year’s lecture.

Still curious if there’s a good explanation for the apparent lack of Neanderthal/Denisovan mitochondrial DNA in modern populations. [I think Paabo still references this in recent versions of his stump talk.]

Is this evidence for F1-onward female-sterility [at least high enough to erase female-lineage hybrids], or strong selection against the N/D versions of mDNA in populations post-hybridizing?

Or was this perhaps a matter the cultural and demographic patterns surrounding the ‘hybrids’ [eg, Neanderthal groups took in any hominine female, but ‘moderns’ accepted male children, not females of mixed-‘marriages’?….

Or just chance — ‘mitochondrial eve’ for Eurasia just missing populations where hybridizing occurred?

Well, our host proposed three possibilities, to which I added fourth and most simple speculation.
If you look at the physique of Neanderthals and Moderns, it is obvious that the former must have been immeasurably stronger (well, not really immeasurably, but very much).
Female Neanderthals were possibly not very attracted to these Modern ‘whimps’ on the one hand, and it would have been a mean task for a Modern ‘whimp’ to rape a strong Neanderthal female.
Conversely, the Modern females might be attracted to this strong Neanderthal, while -again on the other hand- for a Neanderthal raping a Modern human would be a piece of cake.

The first reconstruction of rib cage just showed the same internal volume, with possibly a larger “burst” strength of Neanderthals due to the larger diaphragm bottom of the cage. And they had straighter spine, so presumably not very good at walking or running long distances.

Conversely, it was also recently shown that early hominins routinely had skeletal defects likely due to small local population inbreeding. All of them seem to have been very much the same in physical capabilities.

FWIW Pääbo responded to that in the Q&A. He mentioned pure chance, as if the early mixing was a unique event or – more likely IMO – bottleneck drift (bottleneck of Arabia both in space, time and harsh climate).

The “Elongated metaphase in human apical progenitors” slides around 50:00 in are interesting. I wonder if the metaphase varies among modern human populations at all.

On a separate note, because humans are not isolated from one another, a new species will not evolve. That is what I understand but I’m not sure that’s completely correct. I’m just thinking now, though, that we could freeze some eggs and/or sperm and save them for about 500,000 to 1,000,000 years. Wouldn’t a fertilized egg from a future sperm or a fertilized egg 500,000 years later from a sperm now create something like a mix between the Neanderthals and the Denisovans?

On a separate note, because humans are not isolated from one another, a new species will not evolve. That is what I understand but I’m not sure that’s completely correct.

If you are looking at a model of “allopatric” speciation – where new species develop in geographically isolated populations (“allo-” different ; “-patros” territory, same root as “patriotism”). Which is certainly the best developed of the moels of speciation. But there remain other possibilities such as genetic drift within small, isolated populations (religious cults with strong prohibitions on breeding outside the group are an active range of such groups).
Political isolationism is probably increasing too – I would be utterly unsurprised if isolationist/ survivalist/ differentiating social groups will travel this well-travelled path in the future – if they’re not marching down that road already. Certainly, that fear of “other” is the elephant in the room for an awful lot of the more vocal Brexit people in Britain, with an explicit or implicit policy of trying to get “non-Britons” out of the country. (Not that they ever dare to define “Briton”, because they can’t do that without sounding like the rabid racists that they are. I’m sure President Mushroom-helmet and his KKK/ blue-collar, white-supremacist dominated base will be pushing for development down that line. Have anti-miscegenation laws been reintroduced, yet?

I count that as an infinitesimally small possibility, humans being as they are. It ‘is in their blood’ to miscegenate.
I mean, it was experimented in South Africa (miscegenetion was equated to bestiality!) not long ago, but the population of mixed descent did continue to grow, and not just by people of mixed ancestry reproducing with eachother.
Japanese and Jews are other examples. It did not really work, and nowadays, with so much easy movement of people it is even less likely.
In order to get separate species the near total isolation (allopatric or other) needs at least half a million years, and probably much more (if I understood the work of our host on drosophilia correctly).

This just means that humans won’t split up into multiple new species because they are one population with sufficient gene flow to prevent that. This does not mean that the entire species is not going to gradually change into something new.

Pääbo, besides having an interesting story, also has a very calm, soothing, voice. It makes me feel that things are under control. We are marching toward an intelligent understanding of our human condition. Eventually all problems will be solved. It also is a balm for those of us faced with the voice of chaos on a daily basis.

Thanks for posting this talk. As usual, Svante gives a great talk. I heard him in person a couple of years ago, but the story has grown richer and more complex since then so this was well worth watching.

One point of disagreement, though. His group tends to show the genealogy of individuals by drawing a tree. That’s fine for different species of apes. For showing the relationship between Denisovans, Neandertals, and humans it’s more-or-less OK, as one can superpose crosses between them on a basically treelike genealogy. But it’s not good within each of these groups — the genealogy of multiple humans (or multiple Neandertals or Denisovans) is really an anastomosing networks, owing to migration and recombination. A tree for them can be misleading.

Did not think I’d spend much time on this, but once I started watching I couldn’t stop. Beyond just the fascinating scientific facts that emerge from these studies is the profound nature of science itself. That it not only continues but advances so inexorably and enlightening-ly, that it is so universal, that it challenges our paradigms in so many ways–that it not only tells wonderful stories but provides the bedrock on which we can predict and perhaps improve both our lot as a species and the biosphere itself–is so…phenomenal. (She finishes, lamely.)

And yet there’s the tragedy of so many humans being not only intimidated by science but persuaded that it’s some kind of pernicious, soulless (help me, I can’t come up with a better word ATM), Machiavellian endeavor…

Well, that’s a lot to get out of a nice lecture like this, isn’t it? Forgive my emoting–most inappropriate in such a context.

I am right now working on a text about human evolution, so this lecture is exactly what I need! (Though, given my problems with auditory processing, maybe I’d better search for some articles.)
I am a European and have a positive attitude to my inner Neanderthal, so let me suggest that:

1) Our ancestors were not promiscuous; they were free of prejudice and some of them became star-crossed lovers with members of other populations. (Imagine a prehistoric version of “Scarlet Letter” about a young Homo sapiens female who is ostracized for having a baby with a Neanderthal guy.)

2) I think that, if the Neanderthal genes were harmful because of mismatch with modern genes, natural selection would have purged them by now. So I fancy that those Neanderthal genes that now give us type 2 diabetes are deleterious in today’s conditions, but have helped us survive many cold, hungry winters throughout the millenia.

Thank you! Finally took time to watch, and it was both wonderful and useful.

Pääbo has started to talk about the assimilation possibility which some recent work put up as possible. A bit handwaving on the likelihood though. And only mentioning the population differential, with his examples of “alleles gone bad” implicitly demonstrating how neutral or differently adaptive alleles may work poorly in the larger current population under different adaptive pressures. But his synthesis of sequenced individuals was illuminating.

I cannot agree fully with Pääbo that modern humans are so different – he waved away evidence to the contrary – or that it would necessarily be purely genetic instead of contingent as the null hypothesis is. Dumb luck and persistence has its place, it was the Nth migration out of Africa and he himself mentioned that earlier migrated populations contributed adaptations.